Insulin action in the central nervous system regulates energy homeostasis and glucose metabolism. To define the insulin-responsive neurons that mediate these effects, we generated mice with selective inactivation of the insulin receptor (IR) in either pro-opiomelanocortin (POMC)-or agouti-related peptide (AgRP)-expressing neurons of the arcuate nucleus of the hypothalamus. While neither POMC-nor AgRP-restricted IR knockout mice exhibited altered energy homeostasis, insulin failed to normally suppress hepatic glucose production during euglycemichyperinsulinemic clamps in AgRP-IR knockout (IR DAgRP ) mice. These mice also exhibited reduced insulin-stimulated hepatic interleukin-6 expression and increased hepatic expression of glucose-6-phosphatase. These results directly demonstrate that insulin action in POMC and AgRP cells is not required for steady-state regulation of food intake and body weight. However, insulin action specifically in AgRPexpressing neurons does play a critical role in controlling hepatic glucose production and may provide a target for the treatment of insulin resistance in type 2 diabetes.

Leptin and insulin have been identified as fuel sensors acting in part through their hypothalamic receptors to inhibit food intake and stimulate energy expenditure. As their intracellular signaling converges at the PI3K pathway, we directly addressed the role of phosphatidylinositol 3,4,5 -trisphosphate-mediated (PIP 3 -mediated) signals in hypothalamic proopiomelanocortin (POMC) neurons by inactivating the gene for the PIP 3 phosphatase Pten specifically in this cell type. Here we show that POMC-specific disruption of Pten resulted in hyperphagia and sexually dimorphic diet-sensitive obesity. Although leptin potently stimulated Stat3 phosphorylation in POMC neurons of POMC cell-restricted Pten knockout (PPKO) mice, it failed to significantly inhibit food intake in vivo. POMC neurons of PPKO mice showed a marked hyperpolarization and a reduction in basal firing rate due to increased ATP-sensitive potassium (K ATP ) channel activity. Leptin was not able to elicit electrical activity in PPKO POMC neurons, but application of the PI3K inhibitor LY294002 and the K ATP blocker tolbutamide restored electrical activity and leptin-evoked firing of POMC neurons in these mice. Moreover, icv administration of tolbutamide abolished hyperphagia in PPKO mice. These data indicate that PIP 3 -mediated signals are critical regulators of the melanocortin system via modulation of K ATP channels.

ATP-insensitive KATP channel mutations cause neonatal diabetes mellitus (NDM). To explore the mechanistic etiology, we generated transgenic mice carrying an ATP-insensitive mutant KATP channel subunit. Constitutive expression in pancreatic β-cells caused neonatal hyperglycemia and progression to severe diabetes and growth retardation with loss of islet insulin content and β-cell architecture. Tamoxifen-induced expression in adult β-cells led to diabetes within 2-weeks, with similar secondary consequences. Diabetes was avoided by transplantation of normal islets under the kidney capsule, before induction. Moreover, the endogenous islets maintained normal insulin content and secretion in response to sulfonylureas, but not glucose, consistent with reduced ATP sensitivity of β-cell KATP channels. In NDM, transfer to sulfonylurea therapy is less effective in older patients. This may result from poor glycemic control or lack of insulin, since glibenclamide treatment prior to tamoxifen-induction prevented diabetes and secondary complications in mice, but failed to halt disease progression after diabetes had developed.

Insulin- and leptin-stimulated phosphatidylinositol-3 kinase (PI3K) activation has been demonstrated to play a critical role in central control of energy homeostasis. To delineate the importance of pathways downstream of PI3K specifically in pro-opiomelanocortin (POMC) cell regulation, we have generated mice with selective inactivation of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in POMC-expressing cells (PDK1(DeltaPOMC) mice). PDK1(DeltaPOMC) mice initially display hyperphagia, increased body weight, and impaired glucose metabolism caused by reduced hypothalamic POMC expression. On the other hand, PDK1(DeltaPOMC) mice exhibit progressive, severe hypocortisolism caused by loss of POMC-expressing corticotrophs in the pituitary. Expression of a dominant-negative mutant of FOXO1 specifically in POMC cells is sufficient to ameliorate positive energy balance in PDK1(DeltaPOMC) mice but cannot restore regular pituitary function. These results reveal important but differential roles for PDK1 signaling in hypothalamic and pituitary POMC cells in the control of energy homeostasis and stress response.